When a patient is suffering from atherosclerosis, significant occlusions or blockages are formed on the interior wall of the artery. As a result of these occlusions, the organ or extremity to which blood is to be supplied is compromised and the patient may experience a myocardial infarction or stroke. Some forms of occlusions may be treated by drugs while others require surgery.
In the past, percutaneous transluminal balloon angioplasty (PTBA), dilation of the arterial wall by an inflatable balloon to restore blood flow, was an acceptable means of treatment for this condition and was considered to be a less invasive alternative to surgery. However, PTBA suffers from the disadvantage of a moderate rate of restenosis (the recurrence of blockage). It has been shown that approximately 30% of all patients who undergo balloon angioplasty redevelop the occlusion within three months due to the proliferation of cells lining the vessel wall and other atherogenic processes. Often, the wall of the dilated artery tends to spring back to its original shape following deflation of the dilation balloon. Arterial stenting has been introduced as a solution to this recoil of the vessel wall.
Arterial stenting involves the placement of an expandable coil spring or wire-mesh tube within the occluded artery to reopen the lumen of the blood vessel. One example of an arterial stent is disclosed in U.S. Pat. No. 4,739,762 to Julio Palmaz. The Palmaz device comprises an expandable wire-mesh graft or prosthesis which is mounted upon an inflatable balloon catheter. The catheter assembly, including the graft, is delivered to the occluded area and is then inflated to radially force the graft into contact with the occlusion. As the graft expands, the lumen of the blood vessel is opened and blood flow is restored. After complete expansion of the graft, the balloon catheter is deflated and removed, leaving behind the graft to buttress and prevent elastic recoil of the blood vessel wall.
Although this method is successful in preventing recoil of the vessel wall, restenosis does occur. Smooth muscle cells which form the vessel wall tend to proliferate and build-up in the newly stented area of the blood vessel. This cellular buildup eventually becomes great enough to block the lumen of the blood vessel.
It has recently been determined that localized heating of the blood vessel wall may prevent the proliferation of smooth muscle cells which are believed to cause restenosis. One example of localized blood vessel heating is disclosed in U.S. Pat. No. 4,799,479 to Spears. The Spears patent discloses an apparatus for angioplasty having an inflatable balloon catheter which is provided with a meshwork of electrical wires to supply heat to a vessel wall. Following balloon angioplasty, the external surface of the balloon is heated to fuse together disrupted tissue elements and to kill smooth muscle cells which are believed to lead to restenosis. Unfortunately, the Spears device does not adequately prevent the spontaneous elastic recoil of the arterial wall. Immediately following angioplasty, the arterial wall begins to spring back to its original shape.
Thus stenting in and of itself is ineffective in preventing restenosis due to the occurrence of cellular proliferation. Likewise, balloon dilation in combination with localized heating does not adequately prevent restenosis since the vessel wall tends to spontaneously return to its original occluded shape.
Accordingly, prior to the development of the present invention, there has been no effective treatment of atherosclerosis which also prevents restenosis of the once occluded blood vessel.